Buried heterostructure lasers with semi-insulating blocking layers have low parasitic capacitance, an important requirement for transmitters for high speed optical communication systems.
Typically, a conventional buried heterostructure laser comprises a semiconductor substrate of a first conductivity type, an active region of another semiconductor material capable of emitting light of the required wavelength, e.g., a multiquantum well structure, and an overlying semiconductor layer of a second conductivity type. The active region is typically delineated in a lateral direction by surrounding the active region with a current blocking layer. For example the active region may be defined as a mesa structure, and a semi-insulating blocking layer is grown around sidewalls of the mesa. The blocking layer functions to direct current more effectively through the heterojunction into the active region. A blocking layer is alternatively referred to as a lateral confinement layer or lateral confinement region.
For example, Fe doped InP semi-insulating blocking layers, or lateral confinement layers, grown by metal Organic Chemical Vapour Deposition (MOCVD) have been proposed in the past for buried heterostructure lasers. During growth of InP blocking layers by MOCVD, growth near edges of dielectric masking layers advances more quickly than in the bulk, and results in defects of InP formation that are referred to as "rabbit ear" defects. Good regrowth morphology has been reported for low pressure MOCVD overgrowth of mesa oriented along the [110] direction with minimal mask overhang to prevent formation of rabbit ears, with either sloping {111} B sidewalls or vertical {011} sidewalls. Although growth on the {011} sidewalls was demonstrated by Matsumoto et al in Electronics Letters 30 (1994) 1305, InP growth with good morphology on vertical or re-entrant mesa sidewalls is difficult to achieve.
To achieve more planar regrowth morphology, addition of chlorine containing compounds such as CCl.sub.4 and trichloroethane has been used. A disadvantage of using chlorine containing compounds to improve the growth is the need to adjust the growth conditions when adding these compounds. The growth rate of InP decreases as higher flows of chlorine containing compounds are used. Therefore, to use this method of regrowth the correct flow of chlorine containing compound is determined to get the best regrowth and the flow rate of the indium precursor or the growth time must be increased to compensate. Also, care must be taken to ensure that the addition of chlorine containing compounds does not add any impurities to the grown InP, or it will not be possible to make the InP layer semi-insulating for current blocking.
Thus the present invention seeks to provide a buried heterostructure laser having an improved blocking layer which overcomes or avoids the above mentioned problems.
In summary, the present invention provides a buried heterostructure semiconductor laser in which an active region is contained in a stripe delineated on opposite sidewalls by a current and light blocking layer wherein the blocking layer is a doped InGaAsP alloy, the degree of doping rendering the blocking layer semi-insulating with a bandgap wavelength less than the lasing wavelength of the laser.
According to another aspect, the invention provides a method of fabricating a buried heterostructure semiconductor laser comprising the steps of: a) epitaxially growing on a substrate active material capable of generating light and gain for lasing action; b) selectively etching said active material so as to form a linear mesa active region defined between two linear etched regions; c) regrowing InGaAsP alloy material in the linear etched regions substantially to the height of the mesa, the InGaAsP alloy having a bandgap wavelength less than the lasing wavelength of the laser; d) forming a cladding layer over the mesa and InGaAsP alloy material; e) forming a contact layer over the cladding layer; and f) forming contacts on the substrate and contact layer.
The use of a semi-insulating layer of InGaAsP has already been proposed in relation to circular grating surface emitting lasers which are largely experimental devices. See, for example, C. M. Wu et al, Photonics Technology Letters 4 (1992) 960-963, D. G. Knight et al, Journal of Crystal Growth 134 (1993) 19-28 and U.S. Pat. No. 5,448,581 issued on Sep. 5, 1995 to Wu et al.
In such circular grating lasers, the InGaAsP layer was chosen because it provided good current blocking properties and, more importantly, because its optical properties allowed it to serve as a waveguide. The waveguide function is essential to the operation of this type of laser because light generated in the active region of the laser is continuously propagated through the waveguide and is reflected back through the waveguide from one or more circular gratings until the lasing threshold is reached at which time an output beam is emitted from the surface of the device.
The Knight et al article referred to above showed that the InGaAsP layer could be successfully regrown on vertical mesa sidewalls of the active region. However, there is no suggestion in any of these references of using the InGaAsP layer as a blocking layer in a buried heterostructure laser. Nor would it be readily apparent to a person skilled in the art that this material would make a suitable blocking layer in a buried heterostructure laser primarily because, instead of a waveguiding function as in the circular grating laser, the blocking layer in the buried heterostructure laser must have the exact opposite property, i.e., light blocking capability. Light blocking capability in the current blocking layer is required so that light propagates along and is confined to the active region and eventually emerges from a cleaved end of the active region.
The present inventors are the principal authors of the Knight et al article and the Wu et al article referred to above. Despite there being a naturally widely held belief that a material selected for its light transmitting properties would be unlikely to be useful in an application which required a material with light blocking capability, the present inventors thought it might be worth researching the material for use in buried heterostructure lasers which are commercially much more significant than the circular grating laser.
Experimentation with different compounds of InGaAsP material has proven that, contrary to expectations, a blocking layer formed of InGaAsP can operate satisfactorily in a light blocking capacity in a buried heterostructure laser and, because of its good regrowth properties, is superior to InP and other conventional techniques.